This section provides background information related to the present disclosure which is not necessarily prior art.
Photovoltaic (PV) devices convert sunlight to electricity. A PV device may consist of a single panel, multiple panels, rigid panels, flexible panels, serial panels, parallel panels, etc. The output of a PV device is typically unregulated (i.e. the output varies with changes in sunlight intensity, temperature, etc.). Further, the output of one PV device may differ from the output of another PV device due to manufacturing variations, different operating temperatures, unequal ageing, different positioning and/or mounting angles, different shading from trees, structures or clouds, different amounts of dirt or debris on the respective PV devices, etc.
In the United States of America, safety requirements for grid tied photovoltaic (PV) inverter systems typically limit open circuit PV string voltage to 600 VDC and require the PV panels to be galvanically isolated from the grid. PV systems generally maximize the PV string voltage to keep the operating current level as low as possible. In typical installations, PV string voltage is often limited to 550V to maintain an adequate de-rating margin. Thus, on many energy production days, the open circuit PV string voltage in a typical installation is to be close to 550V.
For most of the available PV panels or modules, the maximum power point (MPP) voltage is in the range of 75% to 85% of the open circuit voltage. Thus, the operating MPP voltage will commonly be in the range of 420V to 460V. The MPP voltage decreases as the operating temperature increases. While delivering relatively high power levels, the temperature of a PV panel increases and the operating MPP voltage falls much lower, commonly to about 350V. In some cases of very high temperature and/or partial shading on the PV panels, the MPP voltage can be as low as 250V. This is common in summer in hot climates. Thus, many commercial grid tied inverters are designed to operate over an MPP voltage range of 250V to 500V.
However, the nominal grid voltage to which a grid tied PV system is tied is often 230V+/−15%. Many inverters employ a buck converter topology and need an input voltage higher than the peak of the grid voltage. Thus, the DC to AC inverter block of a grid tied system typically needs a DC bus of about 400V as a voltage input. As mentioned above, however, the MPP voltage often varies from 250V to 550V and needs to be converted to a bus voltage of approximately 400V.
FIG. 1 shows one prior art grid tied PV inverter system. As shown, the PV string voltage is boosted to a level which is slightly above the maximum expected MPP voltage, 500V in this case. The 500V is then converted into a sinusoidal AC current at the grid frequency and fed into the grid. Because US applications typically require galvanic isolation, an isolation transformer that operates at utility line frequency is used for isolation.
The topology shown in FIG. 1 is often relatively large in size and weight. Further it may be lower in efficiency than some other topologies. A boost converter generally operates at higher efficiency when running on a smaller duty cycle. As the MPP voltage will be mostly in 350V to 400V range, the boost ratio for the system in FIG. 1, and accordingly the duty cycle, is relatively high and the operating efficiency may be relatively low. The inverter efficiency may be degraded by operation at a higher input voltage than needed.
In the prior art grid tied PV inverter system in FIG. 2, the PV string voltage is boosted to a level of about 500V. This DC bus is stepped down to a level of about 400VDC with isolation using a high frequency switching converter and fed in to a grid tied DC to AC inverter. This eliminates the low frequency isolation transformer. The system in FIG. 2 retains a relatively large boost ratio, and duty cycle, in the common operating MPP voltage range of 350V to 450V.
FIG. 3 shows another prior art grid tied PV inverter system in which PV string voltage is reduced with a buck converter to a level slightly below the lowest rated MPP voltage. The PV string voltage is stepped down to about 240V DC. The 240V is then stepped up to 400V DC using a high frequency isolation switching converter stage. This 400V DC bus is fed to the DC to AC grid tied inverter stage. Unlike a boost converter, buck converters operate at higher efficiency with larger duty cycles. The efficiency of the system in FIG. 3 is somewhat limited by the relatively small duty cycles needed to step down the common 350V to 450V MPP voltage to 240V. The isolation stage of the system in FIG. 3 may be complicated by the need to step up the voltage in the isolation stage.